1. Field of the Invention
This invention relates to a process for the production of free-flowing pastes of .alpha.-sulfofatty acid alkyl ester alkali metal salts containing 60 to 70% by weight washing-active substance, in which acidic .alpha.-sulfofatty acid alkyl esters are neutralized with aqueous alkali metal hydroxide solutions under defined pH conditions. In the context of the present invention, washing-active substance (WAS) is understood to be the sum total of .alpha.-sulfofatty acid alkyl ester alkali metal salt and the .alpha.-sulfofatty acid dialkali metal salt--always present as a secondary product--in the neutralized .alpha.-sulfofatty acid ester.
2. Statement of Related Art
.alpha.-Sulfofatty acid alkyl ester alkali metal salts are acquiring increasing significance as surfactants for detergents and cleaning preparations based on renewable natural raw materials. In known processes, the .alpha.-sulfofatty acid alkyl ester alkali metal salts are obtained in the form of aqueous solutions or pastes by neutralization of .alpha.-sulfofatty acid alkyl esters which may be synthesized reaction of lower fatty acid alkyl esters with gaseous SO.sub.3. In the final analysis, the basis for the production of the .alpha.-sulfofatty acid alkyl ester alkali metal salts are fats and oils of natural origin from which the lower fatty acid alkyl esters are obtained by lipolysis and subsequent esterification of the free fatty acids with lower alkanols or by transesterification of the natural triglycerides with lower alkanols. In both reactions, methanol is preferably used as the lower alkanol. The lower fatty acid alkyl esters are mixtures in which C.sub.6-22 fatty acid residues occur, the chain length distribution being dependent on the origin of the natural fats or oils. In many cases, these fatty acid mixtures are not used for the synthesis as such, but rather in the form of certain fractions. Sulfonation of the fatty acid ester mixtures with gaseous SO.sub.3 gives acidic .alpha.-sulfofatty acid alkyl esters which are converted into aqueous pastes of .alpha.-sulfofatty acid alkyl ester alkali metal salts by neutralization to a pH value of 6 to 8. The crude .alpha.-sulfofatty acid alkyl esters and their alkali metal salts are more or less colored products which generally have to be treated with typical bleaches, such as hydrogen peroxide or alkali metal hypochlorite, before and/or after neutralization.
One particular difficulty involved in the production and handling of aqueous .alpha.-sulfofatty acid alkyl ester alkali metal salt pastes arises out of their viscosity behavior in dependence upon the solids concentration. In aqueous compositions, the .alpha.-sulfofatty acid alkyl ester alkali metal salts produced by conventional industrial processes (hereinafter also referred to in short as ester sulfonates) only form solutions or suspensions of such low viscosity that they flow sufficiently freely to guarantee uninterrupted completion of industrial processes at WAS contents of up to about 40% and then again beyond WAS contents of around 55%. In the intermediate concentration range, i.e. at WAS contents of around 40 to 55% by weight, the aqueous ester sulfonate compositions show extremely high viscosity values, assuming the form of more or less solid gels which can neither be stirred nor pumped. In addition, the lower and upper limits of the individual viscosity maxima can vary by .+-.5% by weigh solids content. As a result of this particular concentration/viscosity behavior, ester sulfonate pastes having WAS contents above 35 to 40% by weight cannot be obtained simply by neutralization of the acidic .alpha.-sulfofatty acid alkyl esters with the calculated quantity of aqueous alkali metal hydroxide solutions. After the lower limit to the viscosity maximum has been exceeded, the reacting mixture loses its stirrability and miscibility. The lack of stirrability and miscibility prevents adequate and rapid dissipation of the heat of neutralization. Local concentration and temperature peaks initiate unwanted secondary reactions, more particularly cleavage of the ester bonds present in the ester sulfonates, so that undesirably high concentrations of alkali metal disalts of the free .alpha.-sulfofatty acids are established in the end product. The subsequent processing of ester sulfonate pastes immobilized by the high increase in viscosity is of course also impaired to the point where it is no longer possible solely as a result of the fact that the aqueous compositions in question can no longer be poured or pumped.
The formation of disalts of the free .alpha.-sulfofatty acid alkyl esters is undesirable for several reasons. The disalts show only limited solubility in water and, in addition, have inadequate surface-active properties. Above all, however, disalts as secondary products in ester sulfonate pastes have a considerable viscosity-increasing effect.
There has been no shortage of attempts in the past to eliminate at least most of the unfavorable effects caused by the particular concentration/viscosity behavior of the ester sulfonates and the unwanted formation of .alpha.-sulfofatty acid disalts. Thus, it has been proposed to improve the flow behavior of aqueous ester sulfonate compositions by the addition of flow aids. According to DE-OS 33 05 430, aliphatic alcohols containing 8 to 40 carbon atoms and 1 to 6 hydroxyl groups, alkylphenols and adducts of up to 20 mol ethylene oxide and/or propylene oxide with the alcohols and alkyl phenols mentioned are used as viscosity regulators.
In connection with the unwanted formation of disalts during working up of the acidic .alpha.-sulfofatty acid alkyl esters, DE-OS 31 23 681 describes a process in which the neutralizing treatment is carried out in two steps. In the first step, neutralization is carried out to a pH value of 2.5 to 4 with a 15 to 50% by weight alkali metal hydroxide solution in the presence of a C.sub.1-4 alcohol, preferably methanol, in a quantity of 5 to 20% by weight, based on the weight of the sulfonated product, before a final pH value of 6 to 7 is adjusted in the second neutralization step using a more heavily diluted alkali metal hydroxide solution. It is said to be possible by this process to reduce the disalt content of the ester sulfonate compositions to 5% by weight, based on washing-active substance, or less. A serious disadvantage of this process is obvious: the ester sulfonate pastes produced in this way contain considerable quantities of alcohol which are troublesome in the production of detergent mixtures by spray drying insofar as they can cause unwanted pluming. To limit the alcohol content of the end products, DE-OS 33 34 517 suggests carrying out the optional bleaching step and neutralization of the crude .alpha.-sulfofatty acid alkyl esters in the presence of such a quantity of a lower alcohol that an aqueous slurry containing 30 to 40% by weight and, based on the weight of the .alpha.-sulfofatty acid ester salt, 5 to 15% by weight of a lower alcohol sulfate and 8 to 40% by weight of the lower alcohol is obtained. Finally, the aqueous slurry is said to be concentrated to such an extent that it contains 40 to 65% by weight .alpha.-sulfofatty acid ester salt, 2 to 10% by weight lower alcohol sulfate and at most 2% by weight lower alcohol.
According to DE-OS 34 32 324, the disalt content of .alpha.-sulfofatty acid alkyl ester alkali metal salt pastes can be controlled and reduced by subjecting the crude sulfonation product before treatment with an aqueous medium to a transesterification reaction in which at least 0.5 mol-equivalent alcohol, based on the SO.sub.3 which is not used for the sulfonation, is used. According to DE-OS 35 38 910, sulfofatty acid alkyl ester salt pastes having solids contents above 35% by weight can be produced by subjecting the crude ester sulfonates to transesterification in accordance with DE-OS 34 32 324 and then establishing solids contents of more than 35% by weight in the aqueous pastes during subsequent working by neutralization with or without preliminary or subsequent bleaching.
Processes for the production of .alpha.-sulfofatty acid alkyl ester alkali metal salts in which lower alcohols are added during working up of the crude sulfonation products are attended by the disadvantage that they lead to pasty compositions which cannot be sufficiently lightened with the usual bleaches.